btrfs: sink parameter len to alloc_extent_buffer

Because we're using globally known nodesize. Do the same for the sanity
test function variant.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/extent_io.c
Simple merge

diff --cc fs/btrfs/extent_io.h
Simple merge

diff --cc fs/btrfs/tests/qgroup-tests.c
Simple merge